The present invention relates in general to wireless communications systems and in particular to apparatus, systems and methods for providing signal diversity utilizing multiple antenna beams in such systems.
The performance of wireless communications systems, including analogue, time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA) systems, is adversely affected by interference. One source of interference at the base station is caused by the simultaneous receipt of signals from multiple remote (mobile) units, and in particular when those mobile units are broadcasting on the same frequency. Assuming an ideal antenna and signal propagation conditions, and that the base station is receiving signals of substantially the same power (in CDMA) from each of the mobile units, the level of interference noise is directly proportional to the number of mobile unit signals received at the base station antenna. This problem is compounded when a mobile unit closer to the base station masks the signals received from mobile units a farther distant.
Another type of interference which adversely affects wireless communications systems is caused by multipath effects. In this case, the signal broadcast from a given mobile unit will reflect off various objects in the surrounding environment. As a result, multiple reflected signals taking multiple paths of varying path lengths arrive at the receiver. These multipath components (reflections) arrive at the receiver antenna with varying time delays (phase differences), and depending on the corresponding path lengths, may combine to produce fades in signal strength. In the worst case where multipath signals are received one-half wavelength out of phase, a null can occur due to signal cancellation.
By minimizing interference in the link from the mobile unit to base station the ability to extract data from that signal is improved (i.e. an improved bit-error rate is achieved). Further improvement can be achieved if the gain of the receive antenna is increased or the receiver noise figure is decreased. The most substantial improvements in receiver performance occur if interference minimization is achieved in conjunction with an increase in gain.
The use of diverse antenna locations, i.e., spatially diverse antennas, is useful in providing improved signal reception at receivers, such as the above mentioned TDMA, FDMA, and CDMA receiving systems. For example, analogue advanced mobile phone service (AMPS) and digital advanced mobile phone service (DAMPS) receivers may employ spatially diverse antennas in the receive signal path to provide improved reception of signals. In one such configuration, the receiver receives data from three 120xc2x0 sectors, together providing 360xc2x0 coverage. Each 120xc2x0 sector is covered by two 120xc2x0 antennas with identical views, one antenna feeding the receiver main (or diversity 1) port and the other feeding the receiver diversity (or diversity 2) port. Alternatively, omni-directional antennas may be used to feed a receiver having only a sector and a diversity port. The antennas are typically separated by a predetermined number of wavelengths in order to provide spatial diversity. This spatial diversity ensures that the incoming multipath components from a given mobile unit transmission are substantially uncorrelated.
If the number of required antennas could be reduced, and/or the need to space antennas by substantial distances could be eliminated, a more compact and less complicated base station could be built. Further, if in doing so, interference reduction and gain improvement could also be achieved, the receiver operation could simultaneously be improved.
In sum, the need exists for improved apparatus, systems and methods for receiving signals in a wireless communications system. Such apparatus, systems and methods should reduce interference power and improve receiver gain.
The principles of the present invention allow for multiple antenna beams to be used to feed a smaller number of receiver input ports. Such multiple beams may be provided by a multibeam antenna (or antennas) and/or a plurality of co-located discreet antennas. By using multiple, narrow, beams to focus on selected mobile units, interference can be substantially reduced and antenna gain substantially increased. Similarly, using polarized beams, interference can be substantially reduced through the use of a beam polarized for selected mobile units and/or through the use of combining of alternately polarized antenna beams in order to minimize or avoid destructive combining of received signals. Systems embodying the principles of the present invention can be advantageously applied to wireless communication systems, such as cellular telephone systems and personal communication services (PCS), although such principles are not necessarily limited to these applications.
According to a first embodiment of the present invention, a communication system is provided which includes at least one antenna providing a plurality of antenna beams, ones of which are alternately or differently polarized. Ones of these antenna beams are combined for coupling to a radio, such as a voice receiver radio, or other equipment in order to allow for forming or synthesizing sectors of desired widths from the multiple antenna beams, to provide improved signal quality, or the like.
A first processing branch is preferably included for processing a first plurality of signals associated with first selected ones of the antenna beams. The signal paths are each adapted to receive at least one of the first plurality of signals associated with corresponding ones of the first antenna beams. The first processing branch also preferably includes a combiner for combining the first plurality of signals for coupling to an interface of the radio or other equipment.
A second processing branch is also preferably provided for processing a second plurality of signals associated with second selected ones of the antenna beams. The signal paths are each adapted receive at least one of the second plurality of signals associated with corresponding ones of the second antenna beams. A combiner is also preferably provided for combining the second plurality of signals for coupling to another interface of the radio or other equipment.
In order to better provide combining of uncorrelated signals and, thus, to avoid destructive combining of signals by either of the first and second processing branches, the antenna beams combined in both the first processing branch and the second processing branch are preferably alternately polarized. For example, where the first processing branch combines the signals of four contiguous antenna beams to form a sector of a desired width, a first and third non-adjacent antenna beam to be combined by the first processing branch are each associated with antennas having a first polarization, and a second and fourth non-adjacent antenna beam to be combined by the first processing branch are each associated with antennas having a second polarization. Accordingly, utilizing both angular and polarization diversity, signal orthogonality may be optimized to avoid destructive combining as those antenna beams having a least amount of angular diversity (low order directional orthogonality) are combined having the benefit of polarization diversity (high order polarization orthogonality).
The first and/or second processing branch may include additional circuitry to further decorrelate signals to be combined, such as delays associated with ones of the plurality of signal paths. However, the above described combining of the various antenna beam signals in each processing branch according to the preferred embodiment provides sufficiently orthogonal signals that such delays are not necessary.
It shall be appreciated that the above described interfaces of the radio or other equipment may be interfaces each associated with a different communication or overhead channel, such as a frequency division channel, time division channel, or code division channel. Alternatively, the above described interfaces may be related interfaces, such as a sector and diversity input, associated with a same channel.
The above described alternately polarized antenna beams may be provided by a number of antenna beam arrangements, such as might be provided by an antenna structure adapted to provide substantially non-overlapping antenna beams throughout a desired area of coverage, ones of which being provided with differing polarization. However, a preferred embodiment of the present invention utilizes an antenna structure to provide antenna beams which substantially overlap. For example, according to a preferred embodiment of the present invention, a first set of substantially non-overlapping antenna beams having a first polarization may be provided and a second set of substantially non-overlapping antenna beams having a second polarization may be provided which overlap the first set of antenna beams. According to this preferred embodiment, communications within any particular portion of the area served or illuminated by the antenna structure may be served at least by two antenna beams of varying polarization.
Preferably, the antenna structure includes collocated sets of alternately polarized antenna elements to provide dual polarization in a physical configuration substantially consistent with that of a singularly polarized antenna. Accordingly, the physical aperture, and thus its aesthetic qualities as well as other physical considerations, such as wind loading, remain substantially unaffected as between a polarization diverse antenna of this preferred embodiment and a singularly polarized multibeam antenna of the prior art.
The principles of the present invention provide substantial advantages over the prior art. In particular, multiple antennas or antenna beams may be connected to a receiver which has a number of input ports less than the number of antennas desired. Further, according to the present invention, narrow beam antennas may be used with a receiver to substantially reduce interference and provide increased antenna gain. Further, antennas constructed in accordance with the principles of the present invention do not require substantial, or even precise, spacing between antennas or antenna beam phase centers, as is required in prior art antenna systems to ensure that incoming signals are uncorrelated or do not destructively combine.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.